Microglia are central nervous system resident myeloid immune cells that contribute to neuroinflammation through the release inflammatory molecules, and neural repair through the synthesis of neurotrophic factors. Exposure to early life traumas such as infection or neural injury can prime microglia, resulting in augmented or impaired adult microglial inflammatory responses, potentially increasing susceptibility to infection, cognitive dysfunction, and neurodegenerative diseases later in life. Nothing is known about how neonatal neural insults reprogram microglial activities in adulthood in any CNS injury model. One highly clinically relevant neonatal injury that causes adult cognitive impairments and neuroinflammation is exposure to chronic intermittent hypoxia (CIH), a hallmark of sleep-disordered breathing, which is prevalent in pediatrics (>50% of premature infants and 1-6% of preschoolers). Here, we focus on the molecular mechanisms whereby neonatal CIH exposure initiates changes in microglia that not only prime their inflammatory responses acutely, but also set in motion long-lasting changes that cause exacerbated inflammatory responses to immune challenge in adulthood. Our data suggest that microglial priming is associated with upregulated toll-like receptor 4 (TLR4) levels, an innate immune receptor highly expressed on microglia, and for which endogenous ligands are abundant in the CNS during CIH. Additionally, we find that increased active histone marks at inflammatory gene promoters accompany microglial priming by hypoxia, effects that may be maintained into adulthood. Thus, our overarching hypothesis is that neonatal CIH induces subthreshold levels of inflammation via TLR4 activation that initiates epigenetic histone alterations at primed microglial inflammatory genes, enabling the effects of neonatal CIH to persist into adulthood. To begin to test this idea, we will pursue two specific aims using the shared power of conditional gene deletion in transgenic mice, molecular analyses, and discovery-based gene sequencing (ChIP-Seq). Following neonatal CIH exposure, these aims will: 1) test the requirement for TLR4 signaling in microglial priming both in the neonate and the adult, and 2) evaluate specific histone modifications that are known to contribute to immune cell priming in related myeloid cells, at primed inflammatory gene promoters in neonatal and adult microglia. These are the first studies to investigate the impact of neonatal CIH on adult microglial inflammation, and to identify mechanisms whereby microglia adapt long-term to an early life injury that affects millions of children. They will also provide critical mouse epigenome mapping information for studies seeking to understand the role of chromatin regulation in microglial development and plasticity. Mechanisms elucidated here will have implications for the priming of microglia in many neurological disorders in which intermittent hypoxia and neuroinflammation are co-morbid.
Microglia (resident CNS immune cells) play critical roles in virtually all aspects of adult brain health and disease by performing surveillance, inflammatory, and neuronal support activities, functions that can be detrimentally influenced by early life exposures. Little is known about how normal microglial activities are re-programmed by inflammatory experiences in early neonatal life, or how these effects are able to persist into adulthood. The goal of the present study is to begin to identify and understand the cellular and epigenetic mechanisms underlying neonatal microglial reprogramming so that the activities of these cells can be manipulated for therapeutic benefit.
